The present invention relates to methods, systems and apparatus for use in wireless (mobile) telecommunications networks. In particular, embodiments of the invention relate to retransmission protocols and associated schemes for managing retransmissions of data in such networks.
Third and fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy third and fourth generation networks is therefore strong and there is a corresponding desire in such networks to provide for reliable communications over increasingly large coverage areas.
FIG. 1 provides a schematic diagram illustrating some basic functionality of a wireless telecommunications network/system 100 operating in accordance with LTE principles. Various elements of FIG. 1 and their respective modes of operation are well-known and defined in the relevant standards administered by the 3GPP (RTM) body and also described in many books on the subject, for example, Holma H. and Toskala A [1]. The network 100 includes a plurality of base stations 101 connected to a core network 102. Each base station provides a coverage area 103 (i.e. a cell) within which data can be communicated to and from terminal devices 104. Data is transmitted from base stations 101 to terminal devices 104 within their respective coverage areas 103 via a radio downlink. Data is transmitted from terminal devices 104 to the base stations 101 via a radio uplink. The core network 102 routes data between the respective base stations 101 and provides functions such as authentication, mobility management, charging and so on. Terminal devices may also be referred to as mobile stations, user equipment (UE), user terminal, mobile radio, and so forth. Base stations may also be referred to as transceiver stations/nodeBs/e-NodeBs, and so forth.
One important aspect of wireless telecommunications networks is the provision of retransmission protocols to improve the overall reliability of data transmissions in circumstances where individual transmissions may fail. The provision of appropriate retransmission protocols becomes more significant in circumstances where there is a higher chance of failed transmissions. Accordingly, retransmission protocols can be especially significant where radio propagation conditions are more challenging, for example in respect of a terminal device at the edge of a cell.
In conventional telecommunications networks, including LTE-based networks, retransmission protocols based around HARQ (Hybrid Automatic Repeat reQuest) procedures are often employed. See, for example, Section 5.4.2 in ETSI TS 136.321 v10.6.0 (2012 October) [2] for an overview of HARQ in respect of LTE uplink communications.
The basic principle of hybrid ARQ (HARQ) is that in association with the transmission of a given block of data, for example in uplink from a terminal device (UE) to a base station (eNB), there is sent in the opposite direction (i.e. in this case downlink from the base station to the terminal device) some feedback (acknowledgement signalling) indicating whether the uplink transmission was successfully received. Acknowledgement signalling indicating successful receipt of the associated uplink transmission is commonly referred to as an ‘ACK’ while acknowledgement signalling indicating non-successful receipt of the associated uplink transmission is commonly referred to as a ‘NACK’. In this regard it may be noted that the term “acknowledgment signalling” is used herein as a convenient term for the feedback/response signalling associated with retransmission protocols and the term is used to refer to this signalling generally and regardless of whether the signalling is indicating successful receipt of data (ACK) or unsuccessful receipt of data (NACK). That is to say the term “acknowledgment signalling” is intended to encompass both positive acknowledgement signalling (ACK signalling) and negative acknowledgement signalling (NACK signalling). In this regard acknowledgement signalling may also be referred to as ACK/NACK signalling, feedback signalling and response signalling.
Uplink data will typically include some data bits (systematic bits) and some parity bits associated with forward error correction coding (FEC). If a base station fails to correctly receive a given transmission of uplink data then NACK signalling will be sent back to the UE.
One retransmission approach would be for the UE to retransmit the data using a different combination of systematic and parity bits. In LTE, these different combinations are referred to as redundancy versions (RVs). An eNB receiving such a retransmission comprising a different RV is able to combine the two (or more) RVs in an effort to increase the likelihood of correct decoding. This process is known as an incremental redundancy process.
Another retransmission approach would be for the UE to retransmit the same RV (i.e. comprising the same set of systematic and parity bits). The eNB may then use an approach of maximal ratio combining or similar to optimally combine the two copies of the same data in an effort to increase the likelihood of correct decoding in a process known as Chase combining.
LTE networks provide for the possibility of combining these two principles by a UE first transmitting a sequence of four different RVs (incremental redundancy) without receiving any downlink ACK/NACK signalling followed by a repeated transmission of the four different RVs (permitting Chase combining) if the base station is unable to correctly decode the uplink data from the initial sequence of four RVs. This approach is known as TTI bundling.
A HARQ procedure relying only on Chase combining may require less buffering capability at the receiving end—the eNB in this example—than incremental redundancy since no new systematic/parity bits are received in retransmissions. On the other hand, incremental redundancy increases the likelihood of earlier successful reception at the price of increased buffering.
In accordance with established and well understood LTE principles, a medium access layer (MAC) delivers a transport block (TB) to a physical layer (PHY) for uplink transmission once every transmission time interval (TTI). A TTI's duration corresponds with a subframe, i.e. 1 ms. From the TB, the PHY derives four RVs (discussed further below) which can be used for respective (re)transmissions. In general the RVs may be transmitted in any order, but the default order is RV0-RV2-RV3-RV1.
Thus, in order to communicate a given TB, in one example a UE might first transmit a sequence of data bits corresponding to RV0 in a given TTI. For the purposes of explanation, a sequence of TTIs comprising an uplink radio frame structure may be considered as being sequentially numbered with the first RV for the data being transmitted in TTI 0. If the base station is able to decode the TB from the transmission it will communicate an ACK to the UE. On receipt of an ACK the HARQ process is able to receive the next transport block for uplink delivery. However, if the base station is unable to decode the TB from the transmission it will communicate a NACK to the UE. On receipt of a NACK the HARQ process causes a retransmission of the TB using RV2 to be made. If the base station is able to decode the TB from the combined transmissions of RV0 and RV2 it will communicate an ACK to the UE. If the base station still cannot correctly decodes the TB, it will send another NACK. This will result in the HARQ process retransmitting the TB using RV3, and so on.
To allow time for processing of the received signals and transmission/decoding of the HARQ feedback, in accordance with the LTE standards, for both uplink and downlink, the ACK/NACK signalling for basic HARQ operation is sent in the fourth subframe after an uplink transmission, and any required retransmission are sent in the fourth subframe after that in what is termed a stop-and-wait (SAW) operation.
The basic HARQ timing in this respect is schematically represented in FIG. 2. FIG. 2 schematically shows a series of TTIs for an LTE-based communications network. The TTIs are sequentially numbered 0, 1, 2 . . . and so on with increasing time relative to the TTI in which an initial transmission of RV0 associated with a particular TB is made. Thus, RV0 is transmitted in TTI 0 and the HARQ process governing transmission of the TB waits until TTI 4 to receive a corresponding ACK or NACK associated with this transmission. In the example of FIG. 2, a NACK is received in TTI 4, and so the HARQ process arranges for retransmission using RV2 in TTI 8. Again it is assumed this transmission does not allow the base station to correctly decodes the TB, and so a NACK is received in TTI 12, resulting in the HARQ process arranging for retransmission using RV3 in TTI 16, and so on.
As can be seen from FIG. 2, the majority of TTIs (e.g. TTI 1 to TTI 7 and TTI 9 to TTI 15) play no role in the transmission of the transport block under control of the HARQ process discussed above. In order to increase efficiency a UE may therefore operate eight HARQ processes in parallel with each HARQ process governing transmission of different transport blocks. A first HARQ process operates with uplink data transmissions in TTIs 0, 8, 16, . . . etc., and downlink ACK/NACK signalling in TTIs 4, 12, 20, . . . as schematically shown in FIG. 2. A second HARQ process operates in parallel (but shifted one TTI) with uplink data transmissions in TTIs 1, 9, 17, . . . etc., and downlink ACK/NACK signalling in TTIs 5, 13, 21, . . . etc. Other HARQ processes operate on the other TTIs in a similar manner up to an eighth HARQ process operating with uplink data transmissions in TTIs 7, 15, 23, . . . etc., and downlink ACK/NACK signalling in TTIs 11, 19, 27, . . . etc. Each HARQ process is associated with its own buffer for uplink data.
Thus basic HARQ operation for uplink in a conventional LTE network provides for eight HARQ processes running in parallel and assigned to specific TTIs. This is reduced to four parallel HARQ processes if TTI bundling is employed, as discussed further below. The parallel HARQ processes are maintained by a single HARQ entity in the MAC layer at the UE so that while some HARQ processes stop-and-wait to receive ACK/NACKs, other processes can be transmitting data. Each uplink (UL) HARQ process may only transmit in specific TTIs as indicated above with reference to FIG. 2. If a particular HARQ process has nothing to transmit in a particular TTI to which it is assigned, no other UL HARQ process from that UE can make use of the TTI. This is, in some respects, in contrast to downlink HARQ processes for which there is increased scheduling flexibility for transmissions and any retransmissions.
In the context of an LTE-type network with uplink transmissions on PUSCH (physical uplink shared channel), the base station (eNB) can provide HARQ feedback (ACK/NACK signalling) for a given HARQ process in either of two ways.
A first way is on a PDCCH (physical downlink control channel) in DCI (downlink control information) Format 0 (or 4 with PUSCH Transmission Mode 2). Among the fields the DCI message can contain are the following of relevance to UL HARQ:
a. The New Data Indicator (NDI).
b. The RV to be used.
c. The modulation and coding scheme (MCS) to be used.
d. An UL resource grant.
If NDI is toggled as compared with the last time it was received, the UE determines that it should proceed with transmitting new data (a new TB) in accordance with the rest of the information on PDCCH. If NDI is not toggled, the HARQ process at the UE determines that it should retransmit data associated with the previously transmitted TB using the potentially different RV, MCS, and grant indicated. This approach may be referred to as ‘adaptive synchronous HARQ’, since the MCS can be altered dynamically, but the fixed timeline represented in FIG. 2 must still be adhered to.
A second way is on PHICH (physical HARQ indicator channel). This encodes a single-bit HARQ Indicator (HI) with ‘1’ for ACK and ‘0’ for NACK. PHICH transmission is distributed across the whole system bandwidth in the control region at the start of a subframe in which it occurs. If the UE does not receive a PDCCH containing DCI Format 0 (or Format 4 in PUSCH Transmission Mode 2), it uses the HI decoded from PHICH. If HI=0, the UE retransmits data associated with the previously-transmitted TB using the same MCS and grant as previously, but cycles through the RVs in a predefined order (e.g. RV0-RV2-RV3-RV1), as discussed above. This approach may be referred to as ‘non-adaptive synchronous HARQ’ since the MCS cannot be altered and the fixed timeline represented in FIG. 2 must be adhered to. If HI=1 (corresponding to an ACK), the UE does not retransmit but waits to receive PDCCH before it can continue transmitting. Such a PDCCH could arrive in the same subframe, and would typically have NDI toggled indicating data for a new TB should be transmitted on the associated uplink resource grant provided by the PDCCH.
If there are two spatial layers in use, an independent PHICH is sent for each spatial layer. If carrier aggregation (CA) is in use, there is one PHICH per layer per carrier, and PHICH is sent on the same component carrier as sends the uplink resource grant to which the HARQ feedback corresponds.
Multiple PHICHs can be sent in the same physical resources (in terms of time and frequency), with coinciding (“overlapping”) PHICHs being scrambled by one of eight complex orthogonal Walsh sequences. The resources and sequence are signalled implicitly by the eNB as a function of parameters of the UL resource allocation.
The UE has an RRC configuration limiting the maximum total number of HARQ retransmissions before it must report failure to the Radio Link Control (RLC) layer.
The above has provided an overview of what might be termed “basic” HARQ operation in LTE. A possible drawback with this type of approach is that the delays between retransmissions of RVs associated with a TB which a base station does not correctly decode can introduce significant delays for uplink traffic. This is especially so in circumstances where radio channel conditions are relatively poor. An alternative HARQ mode which aims to reduce these issues is associated with so-called TTI bundling
With TTI bundling the four RVs associated with a given TB are transmitted in four consecutive TTIs but the base station does not send any associated downlink ACK/NACK signalling until four TTIs after the final transmission. The same MCS and grant allocation are used in all four TTIs. This approach quickly provides a base station with all four RVs to improve the likelihood of early correct decoding of the associated TB at the cost of potentially wasteful transmissions (for example if the base station could have correctly decoded the TB from the first two RVs, the transmission of the next two RVs was not necessary). With TTI bundling a single HARQ process governs the retransmission protocol for the bundle of four TTIs in which the four RVs are transmitted.
The HARQ process timings associated with TTI bundling in LTE are schematically represented in FIG. 3. This is similar to, and will be understood from, the above-description of FIG. 2. As schematically shown in FIG. 3, redundancy versions associated with a first transport block, TB1, are transmitted in TTI is 0 to 3 (conventionally in the order RV0-RV2-RV3-RV1). Transmission of the four RVs associated with TB1 is collectively governed by a first HARQ process, H1. Acknowledgement signalling associated with a particular HARQ process for TTI bundling in LTE is transmitted in the fourth TTI after transmission of the last RV of the TB. If the acknowledgement signalling indicates the base station was unable to correctly decode the TB, the HARQ process arranges for the four RVs to be transmitted as another bundle starting in the 13th TTI after transmission of the last RV of the TB in the previous attempt. Compared to “normal” HARQ operation, for example as represented in FIG. 2, this represents a significant increase in potential latency if a base station is unable to decode a TB from a first bundled transmission of RVs.
Thus, referring to FIG. 3, the HARQ process H1 governing transmission of the TB1 waits until TTI 7 to receive a corresponding ACK or NACK associated with the transmission of TB1. In this example NACK signalling is received. The HARQ process H1 therefore operates to retransmit the RV bundle associated with TB1 in TTIs 16, 17, 18 and 19, and awaits associated acknowledgement signalling in TTI 23.
As with non-TTI bundling HARQ operation discussed above reference to FIG. 2, multiple TTI-bundled HARQ processes can operate in parallel with each process governing transmission of a different transport block. Thus, referring to FIG. 3, a second HARQ process H2 is shown controlling transmission of four RVs associated with TB2 in TTIs 4 to 7. In accordance with the defined timings discussed above, acknowledgement signalling for this HARQ process is received in TTI 11, and in this example it is assumed to be an ACK. Accordingly, HARQ process H2 clears its buffer of data associated with TB2 and prepares for transmission of a new TB in the next series of TTIs allocated to HARQ process H2 (i.e. TTIs 20 to 23). Other HARQ processes operate on the other TTIs in a similar manner with a total of four HARQ processes (labelled H1, H2, H3 and H4 in FIG. 3).
As noted above, HARQ processes generally involve (re)transmissions of so-called redundancy versions (RVs) associated with transport blocks for uplink communication on PUCSCH. The forward error correction (FEC) applied for PUSCH in LTE is a rate-⅓ turbo code. Thus the outputs of the FEC process are a stream of systematic bits (corresponding to the TB data for uplink) and two corresponding streams of parity bits. These three streams are individually interleaved and combined to form coded data for a buffer from which the RVs are drawn. The interleaved systematic bits are laid down first, followed by alternating bits from the two parity streams. This process is schematically illustrated in FIG. 4. Working down from the top, FIG. 4 begins with the transport block plus cyclic redundancy check bits (TB+CRC). This is turbo encoded to provide the systematic bits S and the two streams of parity bits P1 and P2. The streams are individually interleaved to generate respective interleaved versions of S, P1 and P2 which are arranged in a buffer associated with the HARQ process responsible for that particular TB in the order discussed above.
The RVs for uplink transmission are created by reading bits out of the buffer from different starting points depending on the RV being used, as schematically indicated in FIG. 4. The number of bits read out for each RV depends on current rate matching and MCS conditions. When the end of the buffer is reached, readout wraps around to the beginning (i.e. it is a ‘circular buffer’). The start point for RV number n is approximately n/4 along the length of the of the buffer from the start plus a fixed offset.
The above description of conventional HARQ operation is primarily focused on an LTE network operating in a frequency division duplex (FDD) mode. HARQ operation for an LTE network operating in a time division duplex (TDD) mode follows boarding the same principles, but with differences in timings associated with the variable arrangement of uplink-only and downlink-only subframes. The HARQ timeline for TDD is altered compared to FDD such that the ACK/NACK signalling arrives, as a general principle, either four subframes after the corresponding UL transmission or at a delay close to four subframes, depending on the uplink/downlink configuration. A similar alteration is made for the timing of retransmissions following receipt of NACK signalling. To make efficient use of resources, the specified number of uplink HARQ processes is different for different uplink/downlink configurations.
For TTI bundling, a bundle size of four TTIs is used for TDD, as with FDD. However none of the available uplink/downlink configurations in current versions of LTE specifications have four consecutive uplink subframes. This means for the set of TDD configurations which support TTI bundling, the four-TTI bundles are not necessarily contiguous in time. Similar rearrangements to the timeline are made with TTI bundling as for normal (non-bundled) TDD HARQ operation.
More details on HARQ operation and the associated aspects of conventional wireless telecommunications systems can be found in the relevant standards.
A drawback with conventional non-bundled HARQ processes is that when channel conditions are relatively poor such that that multiple RVs are typically required to correctly decode a TB, there is a delay of eight subframes between respective RV transmissions, which increases latency. TTI bundling, on the other hand, can provide for rapid incremental combining of the four RVs derived from a TB with potential Chase combining with retransmission(s) of complete bundles as necessary. This works well in circumstances where channel conditions are such that three or four RVs are typically required to correctly decode a TB because the complete set of RVs is transmitted without the delays associated with non-bundled HARQ operation. However, TTI-bundled HARQ processes suffer similar drawbacks where it might not be possible to reliably decode a transport block from a first set of RVs such that Chase combining is typically required. In these circumstances there is a delay of 16 subframes between complete transmissions of the bundled RVs. Furthermore, to implement Chase combining the base station must store the first bundle transmission for 16 subframes, potentially for each of the four parallel HARQ processes. This can introduce a potentially significant buffering requirement at the base station (particularly for high data rates and large transport block sizes) which would apply in each group of resource blocks the eNB is scheduling, and can in some circumstances re-introduce latency that TTI bundling is intended to remove.
The 3GPP Technical Document (TDoc) R1-080443 from 3GPP TSG-RAN WG1 #51-bis Sevilla, Spain, Jan. 14-18, 2008 [3] proposes two ways of arranging HARQ processes such that only one ACK/NACK is sent per TTI bundle, namely with the timeline based on either the first or last subframe of a bundle. This approach uses a different HARQ process in each subframe, splitting the transport block over them in that way. The transmissions are controlled by higher-layer signalling (discussed in more detail in the 3GPP TDoc R2-074889 from 3GPP TSG-RAN WG2 #60 Jeju, Korea, Nov. 5-9, 2007 [4].
The 3GPP TDoc R2-074940 from 3GPP TSG-RAN WG2 #60 Jeju, Korea, Nov. 5-9, 2007 [5] discusses a form of TTI bundling that is close to what was selected for LTE.
In view of the above-identified drawbacks of existing schemes, there is therefore a need for alternative approaches for operating retransmission protocols in wireless communications networks.